Pathway of assembly of ribulosebisphosphate carboxylase/oxygenase from <i>Anabaena</i> 7120 expressed in <i>Escherichia coli</i>

We have placed the genes encoding ribulose-bisphosphate carboxylase/oxygenase from the Anabaena 7120 operon under transcriptional control of the lac promoter carried on the Escherichia coli plasmid pUC19. The genes encoding both the large and small subunit polypeptides ( rbcL and rbcS ) are transcribed and translated so that ≈0.6% of the soluble protein in E. coli extracts is a fully functional holoenzyme with a sedimentation coefficient of approximately 18S, which contains stoichiometric amounts of the two subunits. However, expression of the large subunit polypeptide vastly exceeds that of the small subunit because the majority of transcripts terminate in the intergenic region between the rbcL and rbcS genes. As a result, excess large subunit is synthesized and accumulates in E. coli as an insoluble and catalytically inactive form. Because small subunit is found only in the high molecular weight soluble form of ribulosebisphosphate carboxylase/oxygenase, we propose that the small subunit promotes assembly of the hexadecameric form of the enzyme via heterodimers of large and small subunits.